Method of producing electrical bushings



May 14, 1968 H. BRENNECKE METHOD OF PRODUCING ELECTRICAL BUSHTNGS Filed Sept 10, 1965 INVENTOR Hermann Brennecke ATTOR NEYS United States Patent 3,383,446 METHOD OF PRODUCING ELECTRICAL BUSHINGS Hermann Brennecke, Munchenstein, Switzerland, assignor to Ciba Limited, Basel, Switzerland, a company of Switzerland Filed Sept. 10, 1965, Ser. No. 486,463 Claims priority, application Switzerland, Sept. 17, 1964, 12,106/ 64 14 Claims. (Cl. 264-122) ABSTRACT OF THE DISCLOSURE In a process for forming an electrical bushing an elongate conductor member is aligned in a mould and an electrically conductive hollow cylinder is located in the mould such that it surrounds and is spaced apart from the conductor member by a generally tubular member disposed therebetween. Filler material is then introduced into the mould to at least a level sutficient to support the cylinder in its spaced relationship with the conductor member, the tubular members are withdrawn, and filler material is introduced into the remainder of the mould. Finally, resin is introduced into the mould and hardened to produce the bushing.

The present invention relates to a method of producing electrical bushings in which a conductor extends through the bushing.

In electric insulation bushings there are often installed capacitor inserts for the purpose of controlling the electric field around a conductor extending through the bushing. These inserts are metal bodies in the form of sheets, foils, braided materials or coatings.

When a bushing is made by a wrapping or winding process, the fixing of a capacitor layer presents no difficulty. On the other hand, when a bushing is made of cast resin it is necessary to take special precautions in order to position the capacitor layer as required and to fix it in place.

By a previously known method the capacitor layers are provided in cylindrical cast resin bodies by applying a conductive lacquer to the surfaces of tubes of various diameters, whereupon the tubes are assembled together the one within the other and after that the interspaces are filled by casting under vacuum. This process takes a great deal of time and moreover involves a high percentage of rejects during manufacture due to poor adhesion between the surfaces of the tubes and the subsequently introduced and hardened casting resin. It has been found that bushings of this kind are liable to ionisation under the influence of the voltage applied in operation.

By a further previously known process the construction proceeds from outside inwards. There is first of all cast a tube by centrifugal casting and the internal diameter of this tube corresponds to the external diameter of a cylindrical capacitor insert. This tube is mounted in the casting mould. In the next step of the process the capacitor layer is impregnated with casting resin and then fixed inside the tube. After the resin has hardened there are introduced further inserts and the assembly thus built up from the outside inwards. This process has the disadvantages that the centrifugal casting process is expensive and does not easily allow the desired internal diameter of the cast tube to be obtained precisely.

Turning now to processes in which a bushing is made by loctaing in a predetermined relationship, a conductor and a cylindrical insert in a mould before adding a casting resin, in one such previously known process electrically conducting inserts are prefixed in place by means 3,383,446 Patented May 14, 1968 "ice of electrically insulyating spacers which can for example be made of corrugated paper. This process is relatively simple but has the disadvantage that after the casting resin has hardened there develop fissures between the spacers and the surrounding casting resin. In operation these fissures often give rise after a short time to electric breakthrough due to ionisation. Electric bushings insulated by foam plastics are also unsuitable for high-voltage applications for the same reason.

If there are used absorbent spacers made of paper or the like there is less risk of crack formation, but even in spacers of this kind there develop regions of instability in the dielectric medium and these usually impair the electric properties of the bushing, particularly its highvoltage resistance.

The present invention was developed with the object of mitigating the above disadvantages and in a first aspect the invention provides a process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the conductor member extends are embedded in a resin, comprising the steps of aligning an elongate conductor member in a mould for receiving a resin; locating in the mould an electrically conductive hollow cylinder surrounding and spaced apart from the conductor member by means of a generally tubular member disposed between the conductor member and the cylinder; introducing a filler material into the mould to at least a level sufiicient to support the cylinder in its spaced apart relationship with the conductor member; withdrawing the tubular member from the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

In particular the invention is applicable to producing a bushing in which a central conductor member extends through a plurality of radially spaced apart conductive cylinders coaxial with the conductor member. In this case the cylinders may be spaced apart by a plurality of tubes which nest one in another. Each tube has a cylinder slidably mounted thereon, the tube supporting the inner cylinder being slid onto the conductor member. Filler material is introduced into the mould to support the cylinders, thereby enabling the tubes to be withdrawn from the mould. As the cylinders are held in their required disposition by the tiller material no spacers are required as are needed in the previously known processes mentioned above.

Apparatus for producing electrical bushings comprise a mould; means for alighing an elongate conductor in the mould; a set of tubes adapted to nest together one inside another; a portion of reduced external cross-section on each tube for slidably receiving a conductive hollow cylinder for mounting a plurality of such cylinders in radially spaced apart relationship with respect to said elongate conductor; and means for manipulating said tubes in the mould.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made to the single figure of the accompanying drawing which shows a longitudinal section through a mould in which various components of a bushing are located.

The drawing illustrates an intermediate stage in the production of an electrical bushing and first of all the steps leading to the position shown will be describel. In the first operation a central conductor 1 was aligned by means (not shown) in the floor 3 of a casting mould 2. A hollow metal cylinder 4 was then pushed on a first centering tube 5, and the latter was slid on to the conductor 1. The casting mould was then filled with a grainy filler material F up to a level N1, and this was enough to fix in place the cylinder 4 sufiiciently. After that a second centering tube 6, on to which a second larger diameter,

hollow cylinder 7 had previously been pushed, was pushed over the first centering tube and the mould was filled with the filler F up to the level N2, this being enough to sufiiciently support in place the cylinder 7. If there are more than two cylinders the operations are repeated as described above. The cylinders are provided with flanged ends formed by turning over the rims of the cylinder ends, these flanges aiding in holding the cylinders firmly in the filler material.

A further, outermost cylinder 8 has radial bushes 9 which, in the complete product, provide voltage connections. This cylinder is preferably introduced into the casting mould 2 before the introduction of the centering tube 5, and is attached to the wall of the mould and centered by means of screws 10 inserted through the wall of the mould to engage the bushes 9.

Continuing now from the situation as illustrated in the drawing, in a first variant of the process the mould 2 is entirely filled with filler, whereupon the tubes 5, 6 are withdrawn and further filler F is introduced until the space previously occupied by the tubes has been filled in, whereupon the casting resin is introduced. This variant is used mainly in the case of cylinders whose Walls are impermeable to filler.

According to a second variant of the process the tubes 5, 6 are withdrawn at the same time and at the same rate as the level of the filler F rises in the mould, in such a way that the lower ends of the centering tubes are always just above the surface of the filler. This second variant is used mainly in those cases where the cylinders have apertured walls permeable to the filler. For example, the apertures may be perforation-s in a solid wall or be the spaces in a braided or mesh wall. By this method the openings in the cylinder walls must be larger than the greatest grain diameter of the filler, to prevent any screening effect.

When the mould is finally filled with the filler F, a casting resin is introduced and then hardened to produce the required bushing. These steps are described in greater detail below.

The screws 10 in the bushes 9 of the outermost cylinder 8 are withdrawn only after the casting resin has at least gelled. This is to ensure that no resln can penetrate into the bushes which, as already mentioned, function as electrical connections and in particular as earth connections.

Before the conductor 1 is introduced into the casting mould it is preferably treated as follows. Near the upper end a short length 11, about 5 to 10 cm. is roughened and degreased. The remainder of the conductor surface is then coated with a parting agent 12, for example a silicone oil. Over this parting layer 12 and also on to the roughened surface 11 of the conductor there is then applied an electrically conductive coating 13 which can, for example, be a layer of silver. At the end of the process above described, a conductor treated in this fashion is bonded to the cast resin body firmly and gastightly at the degreased area 11, whereas over the remainder of the length there is left a freedom for sliding movement between the conductor and the resin body, so that longitudinal extensions of the conductor 1 are not transmitted to the resin body. Differences in expansion between the resin body and the conductor 1 may destructively affect the insulation if the two are bonded along the whole length of the conductor. The conductive silver coating 13 prevents ionisation in the boundary layer 12 because the coating is electrically bonded to the conductor at the area 11 and this has the result that the two surfaces of the layer 12 are always both at the some potential.

The tubes 5, 6 and so forth fit slidably within each other at their upper portions 5a, 6a and so forth. As can be seen from the drawing, the lower portions of the tubes are of reduced diameter to accommodate the cylinders, the reduction in radius of the lower portions being greater than the wall thickness of the cylinders, so that when a set of tubes are nested together annular spaces 14 are formed between adjacent tubes to accommodate the cylinders. Alternatively, the upper portions of each tube can have projection guide members around its outer peripheries to slide along the inner surface of the next outer tube.

Preferably there are used to provide the supporting and locating efiect large-grained fillers, particularly quartz and calcite, whose maximal grain size is between 1.2. and 1.5 mm. and there are preferably used hardening casting resin mixtures containing fillers, particularly based on epoxide or polyester resins. The resin mixture can, if de sired, contain a further filler whose average grain size is less than one-fifth and preferably approximately of the average grain size of the filler previously used as a supporting agent. In this way the amount of filler can be increased (-up to 70 to by volume) so that in the final product the insulation body has about the same ooefiicient of thermal expansion as have the conductor and the metal inserts.

The steps of introducing and hardening the casting resin proceed as required with the application of pressure and vacuum. The casting mould, already filled with filler, is closed at the top and, if necessary, evacuated (1 mm. Hg) and filled with an inert gas, for example CO This evacuating and filling process is repeated several times, and then the casting resin mixture is introduced if necessary under pressure (10 atm. g.). For the introduction of the resin the casting mould is provided with the necessary connections in its cover and floor, and by these connections the mould can if desired be connected to a suitable casting apparatus (not shown). The resin is then hardened under the conditions prescribed for the particular mixture. A casting apparatus particularly suitable for the purpose is described and shown in French patent application 1,386,652 (British patent application 5145/64).

The following examples describe resin mixtures and fillers which give particularly good results in regard both to the manufacturing process and to the electrical properties of the completed high-voltage bushing:

Example 1 For locating the cylinders in place and for filling up the casting mould there was used a dried mixture of quartz sands in which the grain sizes were between 0.5 and 2.0 mm.

The casting resin mixture had the following composition:

To parts by weight of a polyglycidyl ether resin liquid at room temperature and having an epoxy content of 5.3 epoxide equivalents per kg. (prepared by reacting epichlorohydrin with bis-(4-oxyphenyl)-dimethylmethane in the presence of alkali) together with 100 parts by weight of hardening agent in the form of a modified mixture of polycarboxylic acid anhydride, said mixture being liquid at room temperature, there were added with agitation, at a temperature of 80 to 100 C. and under a vacuum of 30 mm. Hg, 300 parts by weight of a calcite obtainable under the name Omya BLR-Z (Pluess- Staufer, Oftringen) and having an average particle diameter of 5 microns, and the whole agitated for about another hour. To the resulting mix-ture there were added, after cooling to room temperature and breaking the vacuum, 1.7 parts by weight of tris-(-dimethylamino-methyl-)-phenol as accelerator and 10 parts by weight of a polypropylene glycol known commercially as Polypropylenglykol P 425, of average molecular weight 425. The casting resin mixture was now ready for application.

Example 2 For pre-fixing the cylinder and for filling up the casting mould a quartz sand mixture as in Example 1 was used.

The casting resin mixture had the following composition:

To 100 parts by weight of a polyglycidyl ether resin liquid at room temperature and having an epoxy content of 5.3 epoxide equivalents per kg. (prepared by reacting epichlorohydrin with bis-(4-oxyphenyl) -dimethylmethane in the presence of alkali) together with a hardening agent in the form of 80 parts by weight of hexahydrophthalic acid anhydride, which is a solid .at room temperature, there were added, with agitation at a temperature of 80 to 100 C. and under a vacuum at 1 mm. Hg., 250 parts by weight of a calcite obtainable commercially under the name Ornya BLR-2 (Pluess-Stauier, Oftringen) and having an average particle diameter of 5 microns, and the agitation was continued for about another hour. After cooling to room temperature and breaking the vacuum there was added to the mass thus obtained 1 part by weight of tris-(-dimethylamino-methyl-)-phenol as accelerator. The casting resin was now ready for use.

Example 3 For pre-fixing the cylinder in place and filling up to mould a mixture of quartz sands as in Example 1 was used.

The casting resin mixture had the following composition:

To 100 parts by weight of 9-(3,4-epoxycyclohexyl)-2,3- epoxy-8,l-dioxaspiroundecane together with a hardening agent in the form of 95 parts by weight of hexahydronaphthalic acid anhydride there were added with agitation, at a temperature of 80 to 100 C. and under a vacuum of 1 mm. Hg., 290 parts by weight of a calcite obtainable commercially under the name Omya BLR-2 (Pluess-Staufer, Oftringen) having an average particle diameter of microns, and the whole agitated for a further hour. After cooling to room temperature and breaking the vacuum there were added to the resulting mixture 12 parts by weight of an alcoholate prepared by reacting 0.82 g. of sodium with 100 g. of 3-hydroxymethyl-2,4- dihydroxypentane. The casting resin mixture was now ready for use.

What is claimed is:

1. A process for producing an electrical bushing in which a conductor member extends through a plurality of radially spaced apart cylinders coaxial with said conductive member comprising the steps of coaxially aligning in a mould a central elongate conductor member, a plurality of conductive cylinders and a plurality of tubes disposed alternately with the cylinders to space the latter from each other and from the central conductor member; introducing a filler material into the mould to support the cylinders therein whilst withdrawing the tubes from the mould whereby the positions of the cylinders with respect to the central conductor member are maintained; adding a casting resin into the mould; and hardening the resin to produce a bushing.

2. A process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the member extends are embedded in a cast resin, comprising the steps of aligning an elongate conductor member in a mould for receiving a casting resin mixture; locating in the mould an electrically conductive, apertured, hollow, cylinder surrounding and spaced from the conductor member by means of a tube disposed between the conductor member and the cylinder; introducing a filler material into the mould to at least a level sutficient to support the cylinder in its spaced apart relationship with the conductor member, withdrawing the tube from the mould as the filler material is so introduced; introducing a casting resin into the mould; and hardening the resin to produce a bushing.

3. A process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the conductor member extends are embedded in a resin comprising the steps of aligning an elongate conductor member in a mould for receiving a resin; locating in the mould an electrically conductive hollow cylinder surrounding and spaced apart from the conductor member by means of a generally tubular member disposed between the conductor member and the cylinder; introducing a filler material into the mould to at least a level sufficient to support the cylinder in its spaced apart relationship with the conductor member; withdrawing the tubular member from the mould; introducing filler material into the remainder of the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

4. A process as claimed in claim 3 in which the resin is mixed prior to introduction into the mould, with a further filler material the grain size of which is less than one fifth the grain size of the first-mentioned filler material.

5. A process as claimed in claim 3 in which the resin is mixed, prior to introduction into the mould, with a further filler material the grain size of which is about one hundredth the grain size of the first-mentioned filler material.

6. A process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the conductor member extends are embedded in a cast resin, comprising the steps of coating 2. portion of an elongate conductor member with a parting agent; applying a coating of conductive material over said parting agent and onto said member to form an electrical connection therewith; aligning the conductor member in a mould for receiving a resin; locating in the mould an electrically conductive, hollow cylinder surrounding and spaced apart from the conductor member by means of generally tubular member disposed between the conductor member and the cylinder; introducing a filler material into the mould to at least a level sufiicient to support the cylinder in its spaced apart relationship with the conductor member; withdrawing the tubular member from the mould; introducing filler material into the remainder of the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

7. A process for producing an electrical bushing in which an elongate conductor member extends through a plurality of radially spaced apart conductive cylinders coaxial with the member, comprising the steps of locating an elongate conductor member in a mould; sliding over the conductor member a first tube and a first hollow, conductive cylinder slidably fitted upon the tube and having a portion projecting therefrom the internal aperture through said first tube having a cross-section corresponding to the cross-section of the conductor member to provide a close fit on the latter; introducing a filler material into the mould to a level below said first tube to support the projecting portion of the first cylinder; sliding over said first tube a second tube and a sec ond, hollow, conductive cylinder slidably fitted upon said second tube and having a portion projecting therefrom, the internal aperture through said second tube having a cross-section corresponding to the cross-section of said first tube to provide a close fit on the latter; introducing further filler material into the mould to support the projecting portion of the second cylinder; repeating the last two steps for as many further tubes and conductive cylinders as are required; withdrawing all the tubes from the mould; introducing filler material into the remainder of the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

8. A process as claimed in claim 6, in which further filler material is added into the mould as the tubes are withdrawn to fill the space occupied by the tubes.

9. A process for producing an electrical bushing in which a conductor member extends. through a plurality of radially spaced apart, cylinders coaxial with said conductive member, comprising the steps of coaxially aligning in a mould, a central elongate conductor member, a plurality of hollow, conductive cylinders and a plurality of tubes disposed alternately with the cylinders to space the latter from each other and from the central conductor member; introducing filler material into the mould to support the cylinders therein; withdrawing the tubes from the mould; adding further filler material to occupy the space vacated by the tubes; introducing a casting resin into the mould; and hardening the resin to produce a bushing.

10. A process of manufacturing an electrical bushing having a conductor extending therethrough comprising the steps of making a plurality of tubes having first portions thereof adapted to fit snugly one within the other for locating the tubes coaxially about an axis at increasing radial distances from the axis, and having second portions thereof of reduced diameter compared to the re spective first portions; making a plurality of hollow cylinders of electrically conductive material and having internal diameters corresponding to the external diameters of said second portions to enable a cylinder to be closely fitted onto each second portion and each cylinder having a wall thickness less than the difierence in outer diameter between the first and second portions of the tube onto which it is adapted to fit; aligning in a mould said tubes and the cylinders fitted thereto with each cylinder projecting beyond its associated tube towards the bottom of the mould; introducing a filler material into the mould to support the cylinders therein; Withdrawing the tubes from the mould leaving the cylinders held in position therein by the filler material; introducing a casting resin into the mould to surround the cylinders and the conductor member; introducing filler material into the remainder of the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

11. In a method of locating a plurality of conductive cylinders for an electrical bushing in a mould in a predetermined radially spaced apart relationship with a central conductor member, the steps of sliding a plurality of cylinders of different internal diameters onto tubes having first portions of different external diameters and second portions of greater external diameters than the respective first portions and of difierent internal diameters whereby the second portions may be nested one within another and the cylinders may be supported on the first portions in radially spaced apart relationship; aligning the nested tubes with the cylinders fitted thereto in a mould with the ends of the cylinders near the bottom of the mould spaced progressively further away from the bottom of the mould from the inner to the outer cylinder; and introducing a filler into the mould to support the cylinders therein upon withdrawal of the tubes therefrom.

12. The method according to claim 11 in which the conductive cylinders are provided :with apertures and the filler material has a grain size less than the size of the apertures in the cylinders.

13. A process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the conductor member extends are embedded in a resin, comprising the steps of aligning an elongate conductor member in a mould; sliding over the conductor member a tube and a hollow conductive cylinder slidably mounted on the exterior of the tube and having a flanged end projecting from the tube for positioning near the bottom of the mould when the tube is fitted on the conductor member; introducing a filler material into the mould to cover the flanged end of the cylinder to support the cylinder; withdrawing the tube from the cylinder; introducing filler material into the remainder of the mould; introducing acasting resin into the mould; and hardening the resin to produce the bush mg.

14. A process for producing an electrical bushing in which a conductor member and a conductive cylinder through which the conductor member extends are embedded in a resin, comprising the steps of locating a first electrically conductive, hollow cylinder in a mould by attaching it to the mould; aligning an elongate conductor member so as to extend through said first cylinder, locating in the mould a second electrically conductive, hollow cylinder of lesser diameter than said first cylinder between the latter and the conductor member by means of a tube disposed between the conductor member and said second cylinder; introducing a filler into the mould to at least a level sufiicient to support the cylinder in its spaced apart relationship with the conductor member; withdrawing the tube from the mould; introducing filler material into the remainder of the mould; introducing a resin into the mould; and hardening the resin to produce the bushing.

References Cited UNITED STATES PATENTS 1,315,365 9/1919 Hamm 264262 X 1,403,409 1/1922 Hazelett 264-262 X 3,257,501 6/1966 Sauer 264262 X ROBERT F. WHITE, Primary Examiner.

I. H. SILBAUGH, Assistant Examiner. 

